1. Field of the Invention
This invention relates in general to LSI semiconductor processing and, in particular, to an improved method of processing monolithic integrated circuits having discrete circuit components formed on the obverse side of a common substrate.
As used herein, the expression "monolithic integrated circuits" refers to a single substrate or wafer of semiconductor material, typically monocrystalline, on (or in) which individual active and/or passive circuits are formed which, when appropriately interconnected, result in either a desired network or circuit.
2. Description of the Prior Art
In one co-pending application, assigned to the same assignee as the subject application, filed June 2, 1975, Ser. No. 579,332, now abandoned, there is described a method of forming an n-channel boron-doped polysilicon gate semiconductor device. In that application, the obverse side of a semiconductor substrate wafer is first provided with a layer of silicon dioxide. Thereafter, a silicon nitride layer is placed over the silicon dioxide layer followed by a masking oxide layer which is formed over the silicon nitride layer. A photoresist mask is then placed over the masking oxide layer, after which the masking oxide layer is etched and the unetched masking oxide is used as a mask to etch the silicon nitride-silicon dioxide layers that remain unmasked. The masked areas of the substrate are thus utilized to form the gate areas. The next step of the process requires growing a field oxide on the exposed substrate area after which the now masked portion of the silicon nitride and silicon dioxide is removed to expose the gate area. However, it is this field oxide that is undesirably formed on the reverse side of the substrate.
As a first step toward forming the gate, a gate oxide then is grown over the entire wafer. The remainder of the processing is then continued and additional layers are then either grown or deposited to form the integrated circuit. At the conclusion of the processing of the wafer, and just prior to its being scribed and diced to form the individual chips, the oxide is removed by the previously described process step of placing the reverse side of the wafer in a pool of etchant.
In the above prior art process, after having deposited the gate oxide on the observe side of the wafer, it will be noted that the thickness of the field oxide that has also formed on the reverse side thereof has been increased slightly. However, it should be stated that, in certain instances, the presence of an oxide on the reverse side of the wafer represents no problems. However, when it is imperative to make electrical contact to the substrate, or it is necessary to maintain the substrate at a given reference potential, good engineering practices dictate that contact to the substrate be made on the reverse side in order to conserve space on the obverse side. Accordingly, it becomes necessary to remove the oxide on the reverse side. We propose removing this oxide before the wafer is completely processed so that, after it is scribed and diced the individual chips will provide good electrical contact to the base on which it is mounted.
To accomplish the removal of the field oxide grown on the back of the wafer, the following prior art etching procedure is frequently utilized:
(1) a 1:1 aqueous solution of hydrofluoric acid is poured into a ribbed, plastic Petri dish until the liquid level of the hydrofluoric acid just reaches and flows over the tops of the ribs to form a continuous pool of etchant at the center of the dish; PA1 (2) the dish is then carefully levelled to be certain that the hydrofluoric etchant is maintained at the same level over the entire surface of the dish; PA1 (3) using tweezers the operator very carefully places the wafer in the dish so that only the back or reverse side of the wafer is in contact with the liquid in the dish; and PA1 (4) after an appropriate period of time, determined by the thickness of the oxide on the back of the wafer, the wafer is again very carefully removed from the Petri dish and washed. The wafer is now prepared for scribing and dicing operations to separate the individual chips on the wafer and for the subsequent mounting and affixing of the necessary wires thereto. It should now be obvious that if there is too much acid in the dish, when a wafer is placed therein, the acid will overflow onto the top of the wafer and etch the completed circuits on the obverse side thereof. Conversely, if the operator has put an insufficient amount of acid in the dish, the reverse side of the wafer will not be etched and the oxide will not be removed. In either event, the result will be an inoperative chip since on the one hand the device is destroyed and, on the other hand, remaining or unetched oxide will act as an insulator preventing a proper electrical contact from being made to the back of the chip.